The present invention relates to improved apparatus and methods for operating a downhole actuator system, and more particularly to a downhole actuator system for driving another downhole component with a large actuation force, and still more particularly to a valveless and reversible hydraulic piston actuator system that actuates a piston to drive another downhole member with a large actuation force and then resets the piston to its pre-actuation position.
The conventional practice for actuating a piston downhole under high loads requires a hydraulic actuation system having many component parts, including valves. FIG. 1 illustrates a typical prior art downhole actuation system, which includes a closed hydraulic system 100 having a cylinder 110 with an internal piston 120 disposed therein, a reservoir 130 to store hydraulic fluid, a pump 140 to move the hydraulic fluid, and valves 150, 160 to direct the hydraulic fluid flow. The pump 140 is driven by a prime mover such as an electric motor.
The cylinder 110 includes a high-pressure chamber 112 on one side of the piston 120 and a low-pressure chamber 114 on the other side of the piston 120. The piston 120 includes a shaft 122 that drives another downhole member when piston 120 is actuated. The reservoir 130 is a separate, closed container used to store the hydraulic fluid under high pressure. The pump 140 is disposed between the reservoir 130 and the cylinder 110 such that the pump suction line 142 communicates with the reservoir 130 and the pump discharge line 144 communicates with high-pressure chamber 112. Valve 150, with solenoid or motor operator 152, is located on the pump discharge line 144 leading into cylinder 110. Valve 160, with solenoid or motor operator 162, is located on the reservoir return line 132 connecting the pump discharge line 144 to the reservoir 130. Valves 150, 160 direct the flow of hydraulic fluid between the reservoir 130 and the high-pressure chamber 112, and the fluid path depends upon whether valves 150, 160 are open or closed.
The hydraulic system 100 has three operating configurations: 1) actuating, 2) recirculating, and 3) reverse, determined by the open or closed positions of valves 150, 160. To actuate the piston 120, the operator places the hydraulic system 100 in the actuating configuration by opening valve 150, closing valve 160, and turning on the pump 140. Hydraulic fluid flows along flow path 170 out of the reservoir 130, into the pump suction line 142, through the pump 140, which increases the hydraulic fluid pressure, and into the pump discharge line 144. With valve 160 closed, the flow path into the reservoir return line 132 is blocked, and as indicated by flow arrow 172, the hydraulic fluid flows through valve 150 and into high pressure chamber 112 to actuate the piston 120. As the piston 120 moves forward within cylinder 110, shaft 122 drives a downhole member associated with the piston shaft 122.
To momentarily maintain the piston 120 in a stationary position without shutting down the pump 140, the operator can place hydraulic system 100 in the recirculating position by closing valve 150 and opening valve 160. With valve 150 closed, the flow path into cylinder 110 is blocked, and as indicated by flow arrow 174, the hydraulic fluid flows through valve 160, into the reservoir return line 132, and back to the reservoir 130 along flow path 178. The piston 120 is therefore maintained in a stationary position because hydraulic fluid is prevented from entering or exiting cylinder 110. To maintain piston 120 in a stationary position for a longer duration, the pump 140 can be shut off and both valves 150, 160 closed to prevent hydraulic fluid flow.
To move the piston 120 back up in cylinder 110 and reposition it for another actuation, the hydraulic system 100 is placed in a reverse configuration by opening both valves 150, 160 and shutting off the pump 140. As indicated by flow arrow 176, if the pressure in the reservoir 130 is lower than the pressure in the high-pressure chamber 112, hydraulic fluid will tend to flow out of chamber 112 through valves 150, 160, through reservoir return line 132, and back into reservoir 130 along flow path 178, thereby allowing piston 120 to move upward in cylinder 110.
The conventional hydraulic actuation system 100 of FIG. 1 comprises a complex configuration of parts working together in a closed system. In particular, the valves 150, 160 of the conventional hydraulic actuation system 100 are problematic because they have close internal tolerances and small diameter ports and holes for hydraulic fluid flow, making the valves 150, 160 susceptible to clogging due to small particles entering the valve internals. To ensure the valves 150, 160 do not fail or operate ineffectively, filters and screens are required to prevent small particles from entering the valve internals. It would be advantageous to have a less complex configuration than the conventional hydraulic actuation system 100. In particular it would be advantageous to have a closed hydraulic system that eliminates the need for valves 150, 160 and can also operate bi-directional to actuate a piston under high loads and then reset the piston. It would further be advantageous to have an actuation system that provides a precise movement of the actuation shaft.
The present invention overcomes the deficiencies of the prior art.
The actuator system of the present invention is a valveless, high pressure, positive displacement, axial drive system including a hydraulic fluid reservoir, a hydraulic enclosure, a bi-directional pump assembly driven by an electric motor, and a piston assembly, all contained within an actuator housing. The actuator system may also include a piston repositioning assembly connected to the hydraulic enclosure but separated from the actuator housing.
The hydraulic fluid reservoir maintains fluid communication with the hydraulic enclosure and has an internal compensating piston that adjusts with the changes in fluid level in the reservoir. The hydraulic enclosure encapsulates the pump assembly and piston assembly in hydraulic fluid and provides a closed system that prevents hydraulic fluid contamination, such as by drilling fluids. The hydraulic enclosure includes a low-pressure cavity and a low-pressure chamber above the pump, a high-pressure chamber below the pump, and a piston cylinder defined as the area within which the piston reciprocates.
An electric motor drives the pump and includes electrical conductors, a power section, and a driveshaft. The electrical conductors provide power to the power section. The power section of the motor, which is mounted internally of the actuator housing but outside the hydraulic enclosure, turns the drive shaft, which extends into the hydraulic enclosure. The drive shaft is supported by combination thrust and radial bearings, and there is fluid communication across the bearings so that the motor is exposed to the same pressure as the hydraulic enclosure. Because the drive shaft support bearings do not seal the motor from the hydraulic enclosure pressure, the bearings do not create frictional loses that reduce the force capacity of the system. The lower end of the drive shaft is connected to a linkage, and the linkage is connected to the rotor of the bi-directional pump.
The bi-directional pump assembly preferably utilizes a moineau pump, but can use any type of reversible pump capable of providing adequate pressure to drive the piston with a high actuation force. The pump includes a stator through which the rotor is disposed. The lower end of the rotor extends through a bearing pack, which supports the rotor as it moves. There is a passageway through the bearing pack so that hydraulic fluid can readily flow from the pump to the positioning piston. The lower end of the rotor is threaded into a nut that maintains the bearing pack against a flange in the actuator housing.
The piston assembly includes a positioning piston, a shaft, and a return spring. The positioning piston is connected to a shaft that drives another downhole member when the piston is actuated to move forward, such as, for example, a wedge member of a drill bit steering assembly. The return spring is compressed against the lower face of the positioning piston and provides a reverse force on the positioning piston to move it to its original position after the piston has been actuated.
As an alternative to the return spring in the piston assembly, or in addition thereto, the actuator system may include a piston repositioning assembly that is external to, and positioned in a different axial plane from, the actuator housing. The piston repositioning assembly includes a chamber, a repositioning piston, and a biasing spring. The repositioning piston and biasing spring are disposed within the chamber, which has a port through its wall leading into a fluid passageway that maintains fluid communication between the chamber and the piston cylinder of the hydraulic enclosure. The biasing spring exerts a force on the repositioning piston to force fluid through the fluid passageway and into the piston cylinder. This fluid pushes against the positioning piston to reposition it for another actuation.
The actuator system is typically a component of a downhole tool such as a bottom hole assembly used for drilling the borehole of a well. The actuator system is designed to drive another downhole member, such as, for example, the wedge member of the three-dimensional, steerable drilling assembly of U.S. patent application Ser. No. 09/467,588, hereby incorporated herein for all purposes. However, the actuator system of the present invention may be used for any type of downhole actuator application.
The actuator system is designed to move the piston a precise distance away from the pump to exert a large actuation force. This is achieved by displacing a specific volume of hydraulic fluid from the low-pressure chamber into the high-pressure chamber through the bi-directional pump, preferably a moineau pump. A moineau pump is advantageous due to the reduced pressure drop through the pump components as compared to other pumping equipment. The pressure output from a moineau pump is approximately 150 psi per section of rotor and stator, otherwise known as a stage or lobe. Therefore, a large number of stages will be joined together, end to end, to achieve the required actuation pressure, which is preferably in a range greater than 5000 psi, and more preferably approximately 6000 psi.
The piston responds by stroking a specific distance forward within the piston cylinder to accept the new volume of hydraulic fluid moving into the high-pressure chamber. The pressure increase through the moineau pump determines the pressure or actuation force on the piston, and therefore on the member being driven by the piston. The electrical current that can be passed from the surface down a wireline or through composite coiled tubing to the electric motor is limited. Thus, one objective of the present invention is to maximize the actuation force given the limited current that can be transmitted downhole. Once the piston has been actuated, a linear potentiometer or another device may be used to determine its exact position.
The moineau pump can be driven in either direction, and when the motor is reversed, fluid moves out of the high-pressure chamber, through the pump, and back into the low-pressure chamber. As the pressure is removed from the positioning piston, the return spring will move the piston in reverse into its pre-actuation position. Alternatively, in circumstances such as when the actuator assembly is being tested at the surface, the piston repositioning assembly can provide the force necessary to move the positioning piston to its pre-actuation position.
In summary, the actuator system of the present invention is a valveless, bi-directional, hydraulic piston assembly. The actuator system includes a bi-directional pump assembly designed to accurately displace the piston a given distance with a large force and maximize that actuation force given the limited electrical current that can be transmitted downhole to drive the motor. Because the pump is bi-directional, the piston can be repositioned after actuation by reverse-flowing the pump without using valves, thus eliminating the complexity of some prior art systems.
Thus, the present invention comprises a combination of features and advantages that enable it to overcome various problems of prior art actuation systems. The various characteristics described above, as well as other objects and advantages of the invention, will be readily apparent to those skilled in the art upon reading the following description.